Conventionally, a power source system is known which includes a single DC-DC converter for a battery circuit. This battery circuit is configured so that a fuel cell stack and an electric storage device are connected in series. According to the above conventional power source system, a switching is performed between a plurality of operation modes by adjusting the electric power supply ratio between the fuel cell stack and the electric storage device by altering a switching duty of the DC-DC converter. (See, for example, Japanese Unexamined Patent Application, First Publication No. 2010-104165.)